In solar installations sunlight is converted into alternating current, which can be made available to consumers for example by being fed into a grid. For this purpose, a series circuit of solar modules, referred to hereinafter as a string or a photovoltaic (PV) string, is often connected to a DC input of inverters. By virtue of the series circuit, the string can have a high voltage precisely under intensive insolation, with the result that safety measures have to be implemented in order to ensure, particularly in a fault or hazard situation, that the highest potential difference dropped across the string between two arbitrary points of the string or between one arbitrary point of the string and the ground potential does not exceed a predefined limit value. In this regard, safety standards may prescribe, for example, that in a hazard or fault situation firstly the inverter is disconnected from the grid, and secondly, within a predefined time duration of a few seconds, the highest potential difference dropped across the string is not permitted to exceed the open circuit voltage of an individual solar module, for example 80 V.
While solar installations comprising module inverters in which only a single solar module is connected to the DC input of each inverter fulfill this standard in a manner governed by their design, additional measures are required in the case of string inverters. In this regard, it is known that the standard can be achieved by each solar module being short-circuited, or by a connection line of each solar module being electrically connected to the ground potential. An interruption of all connecting lines between solar modules of a string also leads to compliance with the safety standard in a known manner.
FIG. 1 shows a schematic illustration of a solar installation 1 according to the prior art, wherein the connecting line between adjacent solar modules 4 of a PV string 2 can be connected to ground potential via a drivable switching element 10. An inverter 3 can transmit a close signal to the switching elements 10 via a connecting line 11 if a hazard or fault situation is present, as a result of which at least the inner solar modules of the PV string 2 are short-circuited. Cumulatively or alternatively thereto, it is likewise possible for a connecting line between outer solar modules 4 of the PV string 2 and the inverter 3 to be connected to the ground potential via corresponding switching elements 10 (not illustrated in FIG. 1). In these embodiments, the power supply of the switching elements 10 is likewise affected by the inverter 3. What is disadvantageous here is that additional lines are required for the signal transmission and power supply, and also the necessity of having to design the switching elements 10 for the entire string voltage and the entire string current.
FIG. 2 shows a schematic illustration of a further solar installation 1 according to the prior art. Here each solar module 4 of a PV string 2 is assigned a protective circuit 20, which can be accommodated for example in a connection box of the solar module 4. The protective circuit 20 has an input 23 for connecting a solar module 4 assigned to the protective circuit 20 and an output 24 for connecting further series-interconnected solar modules 4 of the PV string 2. The protective circuit furthermore has a circuit breaker 21, which is arranged between the input 23 and the output 24 of the protective circuit 20 and is able to establish or interrupt an electrical connection between the input 23 and the output 24. Cumulatively or alternatively to the circuit breaker 21, the protective circuit 20 has a short-circuiting switch 22, which is arranged in parallel with the output 24 and which is able to short-circuit the solar module 4 with the circuit breaker 21 closed and to provide a bypass path for the current of the adjacent solar modules 4 within the PV string 2. Energy can be supplied to the protective circuit 20 by energy being drawn from the solar modules 4, with the result that autonomous operation of the protective circuit 20 is possible. At the same time, a drive signal for the protective circuit 20 can be transmitted for example by high-frequency modulation onto the DC string current via the string line (Power Line Communication—PLC). The signal generation can be effected by the inverter 3 to which the PV string 2 is connected. This obviates the need to provide additional lines for the operation of the protective circuit 20 within the solar installation 1. Details concerning the further embodiment of such a protective circuit can be gathered from the document WO 2011/023732 A1.
What is disadvantageous about the protective circuit 20 in accordance with FIG. 2 is the outlay for retrofitting an existing installation, and also the integration of the protective circuit 20 into the connection box of the solar module 4, since a bewilderingly large number of connection boxes exist and the space requirement of the protective circuit 20 or other integration requirements regularly cannot be fulfilled.
Furthermore, the document DE 10 2007 032 605 A1 generally discloses a drivable disconnection box for arrangement between photovoltaic modules. The disclosure says nothing, however, inter alia regarding a supply of the circuit and a process for starting up an installation with disconnected photovoltaic modules.
The document DE 10 2011 050 468 A1 also describes a photovoltaic installation for generating electrical energy with a plurality of photovoltaic modules connected in series. In that case, a switching device is arranged between at least two adjacent photovoltaic modules, by means of which switching device the electrical connection between the adjacent photovoltaic modules can be interrupted.
The document DE 10 2013 106 808 A1 discloses a circuit arrangement for the inline voltage supply of an electrical or electronic device arranged in the region of a line carrying a direct current. The circuit arrangement comprises a parallel circuit comprising two antiparallel-oriented diodes arranged in the line. The voltage dropped across the antiparallel diodes is tapped off by a supply subcircuit. A semiconductor switch for minimizing the power loss of the circuit arrangement can be connected in parallel with the antiparallel diodes.
Finally, the document DE 10 20112 104383 A1 discloses a switching unit for arrangement in a DC line between photovoltaic modules interconnected in series. In that case, a switching element of the switching unit is configured to reduce a current flow of the photovoltaic modules in the DC line. To that end, the switching unit comprises a transmission element in order to couple out an electrical control signal present on the DC line and to control the switching element solely with the energy of the coupled-out control signal. In this case, a coupling element is arranged in parallel with the switching element, which coupling element, with the switching element being nonconductive, conducts the control signal through the switching unit. However, precisely in the case of a large number of series-connected photovoltaic modules or switching units, it is disadvantageous that a significant amount of energy is drawn from the control signal for the energy supply and control of the switching element and, if appropriate, further components of the switching unit. In this regard, on the one hand, only a relatively small power can be transmitted with tenable outlay via the control signal serving primarily for signal transmission. On the other hand, it must be ensured that even after power has been drawn from the control signal and after an associated reduction of the corresponding signal level, the control signal is still reliably received by all the switching units.